Powder coating system

Abstract: An apparatus for applying powder coating material onto large objects such as automotive, truck or other vehicle bodies (32) includes a powder spray booth (12) defining a controlled area (30) within which to apply powder coating material onto the vehicle bodies (32), a powder kitchen (14) located at a remote position from the powder spray booth (12), and a number of feed hoppers (68, 96) located proximate the booth which receive powder coating material from the powder kitchen (14) and supply it to automatically or manually manipulated powder spray guns (42, 46) associated with the spray booth (12). Oversprayed powder coating material is removed from the spray booth (12) interior by a powder collection and recovery system (16) which transmits the oversprayed powder back to the powder kitchen (14) for recirculation to the powder spray guns (42, 46).

Description: Field of the Invention This invention relates to powder coating systems, and, more particularly, to a powder coating system for use in vehicle manufacturing facilities including a powder spray booth, a powder supply located remote from the booth and a powder collection and recovery system. Background of the Invention The application of coating materials to large objects such as automotive and other vehicle bodies has conventionally been accomplished in spray booths having an elongated tunnel-like construction formed with an inlet for the ingress of a vehicle body, a coating application area, a curing or drying area in some designs, and, an outlet for the egress of the vehicle body. In many systems, "conditioned" air, i.e. humidified and filtered air, is introduced by a blower or feed fan into a plenum chamber at the top of the spray booth and then directed downwardly toward the vehicle body moving through the booth. The conditioned air picks up oversprayed coating material within the booth interior and this air entrained oversprayed material is drawn downwardly through the floor or side of the booth by one or more exhaust fans. Filters are provided to capture the oversprayed coating material, and the resulting filtered or clean air is withdrawn from the booth and either exhausted to atmosphere or recirculated within the system for reuse. The coating material in most common use for vehicles such as automobiles, trucks and the like is a high solids, resinous paint material which contains a relatively high percentage of liquid solvent components to facilitate atomization of the resinous material. The problems attendant to the recovery of oversprayed, resinous paint material have been well documented and present a continuing environmental problem for the coating and finishing industry. See U.S. Patent Nos. 4,247,591 to Cobbs, et al. and 4,553,701 to Rehman, et al. As disclosed in U.S. Patent No. 5,078,084 to Shutic, et al., owned by the assignee of this invention, powder coating material has been suggested as an alternative to solvent based liquid paint materials for the coating of large objects such as vehicle bodies. In the practice of powder coating, a powdered resin is applied to the substrate and then the substrate and powder are heated so that the powder melts and when subsequently cooled, forms a solid continuous coating on the substrate. In most powder spraying applications, an electrostatic charge is applied to the sprayed powder which is directed toward a grounded object to be coated so as to increase the quantity of powder which attaches to the substrate and to assist in retaining the powder on the substrate. The application of powder material onto automotive or truck bodies is performed in a spray booth which provides a controlled area wherein oversprayed powder which is not deposited on the vehicle body can be collected. Containment of the oversprayed powder within the booth is aided by an exhaust system which creates a negative pressure within the booth interior and causes the oversprayed powder to be drawn through the booth and into a powder collection and recovery system. The recovered, oversprayed powder can be saved for future use, or is immediately recycled to powder spray guns associated with the powder spray booth. A number of problems are inherent in coating automotive and other vehicle bodies with powder coating material. Due to the design of vehicle manufacturing facilities, the source of coating material is usually positioned at a remote location from the spray booth, i.e. as much as several hundred feet. Moreover, large quantities of powder coating material, e.g. on the order of 300 pounds per hour (136.08 kg per hour) and up, must be transferred from the source to the spray booth over this relatively long distance at flow rates such as 1 to 2 pounds per second (0.45 to 0.91 kg/s). Additionally, the powder coating material must be transferred with the appropriate density and particle distribution in order to obtain an acceptable coating of the powder material on the vehicle bodies. The term "density" refers to the relative mixture or ratio of powder-to-air, and the term "particle distribution" refers to the disbursion of powder particles of different sizes within the flow of air entrained powder material to the spray guns associated with the powder spray booth. It has been found that currently available powder coating systems are generally incapable and/or deficient in transporting large quantities of powder material at high flow rates over long distances, while maintaining the desired density and particle distribution. As noted above, not all of the powder coating material discharged within the powder spray booth adheres to the vehicle bodies moving therethrough. This oversprayed powder material is collected by a powder collection and recovery system at the base of the booth as disclosed, for example, in Patent 5,078,084 to Shutic, et al. In systems of this type, the powder collection and recovery system includes individual groups or bank of cartridge filters each contained within a series of individual powder collection chambers mounted side-by-side beneath the floor of the spray booth. At least one exhaust fan or blower is connected to the individual powder collection chambers by a single plenum. The fan or blower creates a negative pressure within the booth interior, which draws oversprayed, air entrained powder material into each of the individual powder collection chambers where the powder is collected on the walls of the cartridge filters and "clean air" passes therethrough for eventual discharge to atmosphere. Reverse air jets are operated to dislodge the collected powder from the walls of the cartridge filters which then falls to the base of the powder collection chambers where it is removed for collection or recirculation back to the spray guns associated with the powder spray booth. In high volume applications such as coating automotive vehicle bodies, serviceability of the powder collection and recovery system, and, the application of a uniform negative pressure within the booth interior are of particular concern. It has been found somewhat difficult in certain instances to obtain a uniform negative pressure within the booth interior using a single exhaust or blower fan, which, in turn, adversely affects the efficiency with which the powder coating material can be collected and also can disrupt the pattern of powder coating material discharged from the spray guns onto the vehicle bodies moving through the booth. There has also been a need in systems of this type to improve the serviceability of the reverse air jet valves and cartridge filters contained within each powder recovery chamber. Summary of the Invention It is therefore among the objectives of this invention to provide a powder spraying system for applying powder spraying system for applying powder coating material onto large objects such as automotive or other vehicle bodies which is capable of transmitting large quantities of powder material over long distances at relatively high flow rates while maintaining the desired density and particle distribution, which is capable of automatically maintaining the appropriate volume of powder coating material within the system irrespective of demand, which efficiently collects and recovers large quantities of oversprayed powder for recirculation, and, which is comparatively easy to service. In accordance with the present invention, apparatus for applying powder coating material onto objects comprises a powder spray booth, having a ceiling, floor, and opposed side walls defining an interior which provides a controlled area for the application of powder coating material onto objects moving therethrough, powder collection means located immediately adjacent the spray booth for collecting oversprayed powder from the interior thereof, the powder collection means including powder collection units extending along substantially the length of the spray booth, each of the powder collection units having a powder collection chamber for receiving oversprayed powder from the interior of the spray booth; a clean air chamber associated with each of the powder collection units, each of the clean air chambers having an interior which is isolated from the oversprayed powder entering the powder collection chamber; filter means located within each of the powder collection chambers and connected to the clean air chambers for collecting oversprayed powder thereon to form clean air which is introduced into the clean air chambers, a plurality of fan units, each fan unit including a fan plenum which is connected to a fan, each of the fan units connected to a plurality of clean air chambers by a plurality of exhaust ducts, one of the exhaust ducts being connected between each of the clean air chambers and the fan plenum, whereby the fan units create a negative pressure within the clean air chambers connected thereto through the fan plenums and exhaust ducts to draw air entrained powder coating material from the spray booth into the powder collection units. The invention provides an efficient, easily serviceable powder collection and recovery system for the powder spray booth, which produces a uniform, downwardly directed flow of air within the booth interior. The powder collection and recovery system herein is modular in construction including a number of powder collection units extending the length of the powder spray booth and preferably mounted side-by-side beneath its floor. Each of the powder collection units includes a powder collection chamber housing filter means preferably comprising two groups or banks of cartridge filters mounted in an inverted V shape above an angled, fluidizing plate located at the base of the powder collection chamber. A limited number of individual powder collection units are connected by a common duct to a separate exhaust fan or blower unit. Each exhaust fan is effective to create a negative pressure within its associated powder collection units to draw air entrained, oversprayed powder material from the booth interior, downwardly through the floor of the booth and then into each of the powder collection chambers. The oversprayed powder material collects on the walls of the cartridge filters and "clean" air passes therethrough into clean air chambers associated with each powder collection unit. Pulsed jets of air are periodically introduced into the interior of the cartridge filters; from air jet valves positioned thereabove to dislodge powder collected on the walls of the filters which then falls onto the angled fluidizing plate at the base of each powder collection chamber for removal. Each powder collection chamber has an outlet connected to a common header pipe, and a gate valve is positioned in each of these outlet lines. The system controller is effective to sequentially open and close the gate valves so that collected powder material is removed from the various powder collection units in sequence for transfer to the reclaim hopper associated with the powder kitchen. The construction of the powder collection and recovery system herein provides a number of advantages. Because a number of exhaust or blower units are employed, each associated with a limited number of powder collection units, a more uniform and evenly distributed downward flow of air is created within the interior of the powder spray booth along its entire length. This is an improvement over systems having a single exhaust fan or blower because it has proven difficult to obtain a uniform negative pressure within a spray booth having the extreme length required to coat large objects such as vehicle bodies with only one blower unit. Servicing of the powder collection and recovery system herein is also made much easier than in prior designs. The reverse air jet valves are located at the top of the powder collection units for easy access, and the cartridge filters are easily removed from the powder collection chambers by one operator. Removal of powder material from each of the powder collection chambers is also made easier by the angled, fluidizing plate at the base thereof which aids in smoothly transferring powder out of the chambers. Additionally, the walls of the powder collection chamber are made sufficiently thin so that they are vibrated when the reverse jets of air are activated to assist in the transfer of powder onto the porous plate. Particularly preferred embodiments of the apparatus are the subject of parent European Patent Application No. 0701486. Description of the Drawings The structure operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein: Fig. 1 is a partial schematic view of one end of the powder spray booth including feed hoppers and a portion of the powder collection recovery system, and including a schematic depiction of the powder kitchen; Fig. 2 is an elevational view of a powder receiver unit and primary hopper contained within the powder kitchen; Fig. 3 is a plan view of the primary hopper shown in Fig. 1; Fig. 4 is a cross-sectional view taken generally along line 4-4 of Fig. 3; Fig. 5 is an elevational view in partial cross-section of a feed hopper for use with the booth of Fig. 1; Fig. 6 is a schematic, partially cut-away view of a robot hopper for use with the booth of Fig. 1; Fig. 7 is a schematic, partially cut-away view of the powder collection and recovery system herein; Fig. 8 is an end view of a powder collection chamber; and Fig. 9 is a side view of the powder collection chamber depicted in Fig. 8. Detailed Description of the Invention Referring now to the Figs., the powder coating system 10 includes a powder spray booth 12, devices for transferring powder coating material from a powder kitchen 14 to the booth 12, and, a powder collection and recovery system 16 associated with the booth 12. The powder spray booth 12 and the powder collection and recovery system 16 are described separately below. The other elements of the powder coating system 10 including powder receivers 58, 66, 82 and 94, primary and reclaim hoppers 60 and 80, feed hoppers 68 and 96 and robot hoppers 78 shown in the drawings are all described in detail in the above referenced parent European Patent Application 0701486. Powder Spray Booth Referring to Figs. 1 and 2, the powder spray booth 12 includes a ceiling 18, floor 20, opposed side walls 22, 24 and opposed end walls defining a booth inlet 26 and a booth outlet 28. See also Fig. 7. This construction of spray booth 12 defines an interior 30 forming a controlled area in which to apply powder coating material onto objects such as a vehicle body 32 moved by a conveyor 34 through the longitudinally extending center portion 36 of the spray booth 12. Oversprayed powder material which does not adhere to the vehicle body 32 passes through gratings 38 located along the floor 20 of spray booth 12 and into the powder collection and recovery system 16 described in detail below. The powder spray booth 12 extends for a substantial longitudinal distance, and can be provided with a variety of powder spray guns positioned at different locations therealong so that all areas of the vehicle body 32 are coated with powder coating material in the course of passage through the booth interior 30. For purposes of illustration, a robot 40 carrying a spray gun 42 is depicted on one side of the spray booth 12, and an overhead gun manipulator 44 is illustrated in position above the vehicle body 32 carrying spray guns 46. Depending upon the size of the vehicle body 32, the types of powder coating material to be applied thereto, the desired areas of coverage on the vehicle body 32 and other factors, essentially any number of spray guns manipulated either automatically or manually can be provided along the length of the spray booth 12 for covering the vehicle body 32 with powder coating material. The particular location and operation of such spray guns forms no part of this invention of itself, and is therefore not discussed herein. In the presently preferred embodiment, the vehicle body 32 is held at ground potential by the conveyor 34 and an electrostatic charge is imparted to the powder coating material by the spray guns 42 and 46. The electrostatic charge applied to the powder material increases the quantity of powder which adheres to the vehicle body 32, and assists in retaining powder thereon, but a relatively large quantity of powder material is nevertheless "oversprayed", i.e. fails to adhere to the vehicle body 32. This oversprayed powder must be collected and recovered in the course of the powder coating operation, as described below. Powder Collection and Recovery System With reference to Figs. 1 and 7-9, the powder collection and recovery system 16 is illustrated in. further detail. This system 16 is generally related to that disclosed in U.S. Patent No. 5,078,084 to Shutic, et al. As noted above, the powder collection and recovery system 16 is located below the floor 20 of powder spray booth 12 on either side of the center portion 36 of booth 12 along which the vehicle bodies 32 are transported by conveyor 34. As depicted at the left hand portion of Fig. 7, gratings 38 cover the booth floor 20 so that oversprayed, air entrained powder coating material can be drawn downwardly from any area within the booth interior 30 into the system 16. The powder collection and recovery 16 is modular in construction and generally comprises a series of powder collection units 346 mounted side-by-side and extending longitudinally along the entire length of the booth 12. See center of Fig. 7. The powder collection units 346 are connected in groups of three or four, for example, to individual fan or blower units 348 located beneath the powder collection units 346, as shown in Fig. 1 and the right side of Fig. 7. Each of the powder collection units 346 includes a collector housing 350 having opposed side walls 354, 356, opposed end walls 358, 360 and an angled or sloped bottom wall 362. A clean air chamber 364 is located at the top of collector housing 350 which is formed by a pair of inwardly angled support plates 366, 367 each having a number of spaced openings 368, opposed side plates 369, 370, and, a pair of access doors 371, 372 which are hinged to the side plates 369, 370, respectively. The clean air chamber 364 extends across the length of collector housing 350 and connects to an extension 373, the purpose of which is described below. The lower portion of collector housing 50 forms a powder collection chamber 374 having tapered sidewalls and a bottom wall defined by a porous plate 376. The porous plate 376 is mounted above the base 362 of collector housing 350, at an angle of approximately five degrees with respect to horizontal, which forms an air plenum 377 therebetween. An upwardly directive flow of air is introduced into the air plenum 377 beneath the porous plate 376 through an inlet (not shown) so that powder coating material entering the powder collection chamber 374 is fluidized atop the porous plate 376. In the presently preferred embodiment, two groups or banks of cartridge filters 378 are located within the powder collection chamber 374 and are arranged in an inverted V shape as seen in Fig. 8. The open top of each cartridge filter 378 is carried by one of the support plates 366, 367 of clean air chamber 364 in position over an opening 368 in such plates 366, 367. Each cartridge filter 378 has a central rod 382 threaded at its upper end to receive a mount 384 which is tightened down on the rod 382 such that one of the support plates 366 or 367 is sandwiched between the mount 384 and the top of a cartridge filter 378. Preferably, one or more filter mounting plates 386 extending between end walls 358, 360 of collector housing 350 provide additional support for each cartridge filter 378. In order to dislodge powder coating material from the walls of the cartridge filters 378, which enters the collector housing 350 as discussed below, a set or group of air jet nozzles 392 is provided for each bank of cartridge filters 378. One set of air jet nozzles 392 is carried on a nozzle support 394 mounted within clean air chamber 364, and the second set of air jet nozzles 392 is carried on a nozzle support 396 within the clean air chamber 364. As depicted in Fig. 8, each set of air jet nozzles 382 is aimed at the open tops of one group or bank of cartridge filters 378. The air jet nozzles 392 associated with each bank of cartridge filters 378 are connected by air lines 398 to a pneumatic valve 400, which, in turn, is connected to the source 73 of pressurized air. In response to a signal from the system controller 108, the pneumatic valves 400 are operated to selectively direct pressurized air through air lines 398 so that a jet of pressurized air is emitted from the air jet nozzles 392 into the interior of one or both of the banks of cartridge filters 378. These pulsed jets of air dislodge powder coating material from the walls of the cartridge filters 378 so that it can fall by gravity into the powder collection chamber 374 and, onto the porous plate 376. With reference to Figs. 1 and 7, air entrained powder coating material is drawn into each of the powder collection units 346 from the booth interior 30 under the application of a negative pressure exerted by the blower units 348 mentioned above. Each of the blower units 348 includes a fan plenum 402 which houses a fan or blower 404 and a number of final filters 406 depicted schematically in Fig. 1. The fan plenum 402 is formed with a number of openings 408 over which an exhaust duct 410 is fixedly mounted. Each exhaust duct 410 extends vertically upwardly into engagement with a coupling 412 located at the base of one of the extensions 373 of the clean air chambers 364 associated with each powder collection unit 346. In response to the operation of blower 404 within fan plenum 402, a negative pressure is developed within the exhaust duct 410 and, in turn, within the clean air chamber 364 associated with each of the powder collection units 346. This negative pressure creates a downwardly directed flow of air in the booth interior 30 within which oversprayed powder coating material is entrained. The air entrained powder coating material passes through the gratings 38 at the floor 20 of the spray booth 12 and enters each of the powder collection units 346 where the powder coating material is collected along the walls of the cartridge filters 378 or falls onto the porous plate 376 at the base of collector housing 350. An important feature of the powder collection and recovery system 16 is that one blower unit 348 services a limited number of powder collection units 346. For example, the blower unit 348A depicted on the right hand portion of Fig. 7 has a fan plenum 402 formed with four openings 408 each of which receive an exhaust duct 410 connected to one powder collection unit 346. Accordingly, four powder collection units 346 are accommodated by one blower unit 348A. Other blower units 348 are associated with relatively small groups of adjacent powder collection units 346 which results in the application of a uniform, downwardly directed flow of air throughout the booth interior 30. Further, the configuration of the clean air chamber extensions 373 of each powder collection unit 346 permits the powder collection units 346 on one side of spray booth 12 to "dovetail" or fit closely adjacent the powder collection units 346 on the opposite side of booth 12. See center of Fig. 7. This conserves space and reduces the overall dimension of the booth 12. Another feature of the powder collection and recovery system 16 is the retrieval of collected, oversprayed powder from the powder collection units 346 for recirculation back to the powder kitchen 14. As mentioned above, air entrained powder material from the booth interior 30 is drawn into each of the powder collection units 346 and falls either by gravity onto the porous plate 376 at the base thereof or is dislodged from the walls of the cartridge filters 378 by periodic bursts of pressurized air emitted from the air jet nozzles 392. In the presently preferred embodiment, movement of the powder onto the porous plate 376 is assisted by the forming of the walls 354-362 of the collector housing 350 of each powder collection unit 346 of a relatively thin gauge metal, such as 18-20 gauge No. 304 stainless steel, so that they vibrate when the reverse jets of pressurized air are emitted from air jet nozzles 392. Because the porous plate 376 is angled at about five degrees with respect to horizontal, the fluidized powder coating material thereon flows toward an outlet 422 on one side of the collector housing 350 at the lower end of porous plate 376. In turn, each of the outlets 422 of powder collection units 346 is connected by a branch line 424 to a common header pipe 426 which extends longitudinally along the length of powder booth 12 on both sides thereof. The header pipe 426 is connected to the reclaim line 86 which leads to the third powder receiver 82 within the powder kitchen 14. Preferably, a guillotine-type gate valve 428 is carried within each branch line 424, and these valves 428 are movable between an open position to permit the flow of powder coating material therethrough and a closed position to prevent such flow. In response to activation of the third vacuum pump 84 within the powder kitchen 14, which is associated with third powder receiver 82 and reclaim hopper 80 as described above, a negative pressure is produced within the header pipe 426. The system controller 108, mentioned above in connection with the powder transfer system, is operative to selectively open the gate valves 428 associated with each powder collection unit 346 so that the powder therein is drawn through their respective branch lines 424 into header pipe 426. Because of the large number of powder collection units 346, only a predetermined number of gate valves 428 are opened at any given time to limit the total amount of powder material which is allowed to enter the header pipe 426 for transfer to the reclaim line 86 leading to the third powder receiver 82 and reclaim hopper 80. With reference to Fig. 1, a pressure sensor 430 is schematically depicted as being connected to the fan plenum 402 of the blower unit 348. The purpose of pressure sensor 430 is to sense the pressure drop across final filters 406 within blower unit 348 and send a signal representative of same to the controller 108. In the event of a failure or other problem with one or more cartridge filters 378 within the powder collection unit 346 associated with a given blower unit 348, the passage of powder coating material into the clean air chamber 364 and then to the final filters 406 creates a pressure drop across the final filters 406. This pressure drop is sensed by the pressure sensor 430 at which time a signal representative of such pressure drop is sent to the controller 108 to alert the operator of a problem within such powder collection unit 346. Because there are a number of blower units 348, each associated with a group of powder collection units 346, a failure within the powder collection and recovery system 16 can be pinpointed and attributed to one blower unit 348 and an associated group of powder collection units 346. This facilitates maintenance of the system and avoids the operator having to check each of the blower units 348 for such problems.

Claims: Apparatus for applying powder coating material onto objects comprising a powder spray booth (12), having a ceiling (18), floor (20), and opposed side walls (22, 24) defining an interior (30) which provides a controlled area for the application of powder coating material onto objects moving therethrough, powder collection means (16) located immediately adjacent the spray booth (12) for collecting oversprayed powder from the interior (30) thereof, the powder collection means including: (i) powder collection units (346) extending along substantially the length of the spray booth (12), each of the powder collection units having a powder collection chamber (374) for receiving oversprayed powder from the interior of the spray booth (12); (ii) a clean air chamber (364) associated with each of the powder collection units (346), each of the clean air chambers (364) having an interior which is isolated from the oversprayed powder entering the powder collection chamber (374); (iii) filter means (378) located within each of the powder collection chambers (374) and connected to the clean air chambers (364) for collecting oversprayed powder thereon to form clean air which is introduced into the clean air chambers (364), a plurality of fan units (348), each fan unit (348) including a fan plenum (402) which is connected to a fan (404), each of the fan units (348) connected to a plurality of clean air chambers (364) by a plurality of exhaust ducts (410), one of the exhaust ducts (410) being connected between each of the clean air chambers (364) and the fan plenum (402), whereby the fan units (348) create a negative pressure within the clean air chambers (364) connected thereto through the fan plenums (402) and exhaust ducts (410) to draw air entrained powder coating material from the spray booth (12) into the powder collection units (346). Apparatus as claimed in Claim 1 in which each of the powder collection units (346) is connected by a connector line (424) to a common header pipe (426), each of the connector lines (424) carrying a valve (428) moveable between an open position and a closed position. Apparatus as claimed in Claim 2 further including means (84) for applying a negative pressure within the common header pipe (426), and means (108) for sequentially opening and closing the valves (428) associated with each of the powder collection units (346). Apparatus as claimed in any preceding Claim in which each of the fan plenums (402) includes a number of final filters (406) and means for sensing the pressure drop across the final filters (406). Apparatus as claimed in any preceding Claim in which the powder collection chamber (364) of each of the powder collection units (346) includes a porous plate (376) mounted at an angle relative to horizontal in position to receive powder coating material from the spray booth (12), the powder collection chamber being formed with an air inlet for directing a flow of air through the porous plate (376) to fluidize powder coating material received thereon. Apparatus as claimed in any preceding Claim in which the filter means includes two banks of cartridge filters (378) mounted in an inverted V-shape within the powder collection chamber (364) of each powder collection unit (346). Apparatus as claimed in Claim 6 further including a number of reverse air jet valves (392) mounted within the clean air chamber (364) of each powder collection unit (346) in position above the cartridge filters (378), each of the reverse air jet valves being positioned in alignment with one of the cartridge filters.

Inventors: CARLSON JOHN F, FENA ERNEST J, HOLLSTEIN THOMAS E, SHUTIC JEFFREY R, WILLIAMS KEITH E, CARLSON, JOHN F., FENA, ERNEST J., HOLLSTEIN, THOMAS E., SHUTIC, JEFFREY R., WILLIAMS, KEITH E.
Applicants: NORDSON CORP, NORDSON CORPORATION
Application year: 1994
Main classification: B05B5
IPC codes: B05B7, B05B15